Bentonite deposit has a complicated composition mainly formed of smectites and containing free silica minerals, such as quartz, .alpha.-cristobalite, and opal; silicate minerals, such as feldspar, mica, and zeolite; carbonates or sulfates of alkaline earth metals, such as calcite, dolomite, and gypsum; and, in addition, iron compounds and humus. Since bentonite ore mined from bentonite deposit usually has a water content of 15 to 35%, it is primarily broken and dried in the sun or hot air to obtain bentonite ore having a water content of 5 to 10%. The dried bentonite ore is pulverized by means of an attrition grinding machine, such as a centrifugal roller mill, or an impact mill, such as a hammer mill. The resulting bentonite powder is widely used in various industrial fields, for example, as a binder for foundry sand, a main ingredient for a drilling fluid used in oil well drilling, geothermal well drilling, or hot spring boring, or a main agent of a stabilizing liquid used in continuous diaphragm wall construction or earth drilling in engineering works. As stated above, bentonite for these uses basically has a composition based on the natural bentonite deposit, containing much non-clay substances.
On the other hand, liquid fine chemicals widely used in various industrial fields, such as coatings, printing inks, and cosmetics, contain various natural or chemical substances for rheological adjustment. Purified bentonite powder, which is obtained by dispersing bentonite in water, removing unfavorable non-clay substances by spontaneous sedimentation or centrifugal separation, and drying the resulting purified bentonite sol by evaporation, is used as a rheological adjuster for aqueous coatings, aqueous emulsions, or the like aqueous colloidal dispersion products.
Because the bentonite powder which is merely purified can not be dispersed in organic-solvents, it is not suitable as a rheological adjuster for liquid products containing an organic solvent, such as alkyd resin coatings or other synthetic resin coatings, printing inks, and sealants. Hence, the purified bentonite is rendered organophilic by combining with a quaternary ammonium cation (see U.S. Pat. No. 2,531,427), a composite of a quaternary ammonium cation and a nonionic organic compound (see Japanese Patent 244306 corresponding to JP-B-3018 (The term "JP-B" as used herein means an examined Japanese patent publication), or a combination of a quaternary ammonium cation and an organic anion (see JP-A-57-111371), (the term "JP-A" as used herein means an "unexamined published Japanese patent applications") to be used as a rheological adjuster which can be dispersed in organic liquids.
However, the above-mentioned purified bentonite or organophilic modified bentonite has disadvantages as described below.
Purified bentonite exhibits unique rheological characteristics which differ from that of organic (high) polymers, e.g., carboxymethyl cellulose, poly(sodium acrylate), and polyacrylamide. However, because its function in increasing viscosity is generally insufficient, it must be added in a considerable proportion to obtain a sufficient effect in increasing viscosity, which leads to generation of various undesired side effects, such as impairment of the color tone of products. Accordingly, the amount of the purified bentonite incorporated is limited.
Organophilic modified bentonite has insufficient dispersibility in a solvent system comprising hydrocarbons only, and it is necessary to add to the dispersion system an adequate amount of a highly polar organic compound having a low molecular weight, such as methanol, ethanol, or acetone, to obtain organophilic modified bentonite which is sufficiently dispersed. However, such usage is not only complicated but involves a problem that an unfavorable polar compound should be introduced into the system.